DOF of FF and 1.6 crop

I just learned something new today, and I would like to verify this information with someone who has a good understanding of DOF.

I always thought FF had a shallower DOF than 1.6 crop, and it certainly is shallower if you are trying to maintain a same subject size to a frame size ratio, but if you ignore that ratio and keep the distance to the subject the same, then the 1.6 crop is in fact has a shallower DOF than FF, due to the smaller circle of confusion value on 1.6 crop.

For an instance, let's say that I used 100mm f/2.0 lens on the 5D, and I kept the distance of 10m to my subject to frame my shot.
5D's total depth of field from near limit to far limit would be 1.19m.

If I am to use my 20D to frame the same shot using the same exact lens, I would have to increase the distance to the subject by 6m to make it total distance of 16m, then I would have same subject to frame ratio size as prior shot.
And 20D's total DOF would be 1.94m, which is deeper of course.

However.. if I would have ignored the subject to frame ratio size and kept the distant to the subject to be same 10m, then the 20D's total DOF would be 0.75m, which is shallower.

To make it similar DOF as 5D, I would only have to increase the distance to my subject by 2.5m to make it total of 12.5m, then it yields DOF of 1.18m.
And I find that fascinating.

To calculate this, I used online DOF calculator from here http://www.dofmaster.c​om/dofjs.html.

Any remark would be appreciated.
Specially on the CoC calcuation.
Thanks.

Pasukun wrote in post #11727368
I always thought FF had a shallower DOF than 1.6 crop, and it certainly is shallower if you are trying to maintain a same subject size to a frame size ratio, but if you ignore that ratio and keep the distance to the subject the same, then the 1.6 crop is in fact has a shallower DOF than FF, due to the smaller circle of confusion value on 1.6 crop.

What you learned is true, but only of academic interest. The cause is related to the CoC but is best thought of in terms of enlargement from sensor to print.

To make a print of a given size a smaller format must be enlarged more than a larger format. This increased enlargement causes a smaller DOF in the print on from the smaller format.

The reason that this is academic is that nobody uses cameras this way. You don't take a photo at 35mm and f/5.6 on a 1/1.7' sensor P&S and then use the same position, focal length and aperture on a 1.6X or FF camera. 35mm is a long telephoto on a 1/1.7" sensor, normal on 1.6X and a wide angle on FF.

To take the same photo (same perspective, same framing) with different formats will require the use of a longer lens on a larger format. The DOF effect of the longer focal length totally swamps the smaller (and directionally opposite effect) of the enlargement. So functionally, larger formats make for less DOF when compared practically.

Here's the most recent discussion of this topic...

https://photography-on-the.net/forum/showthre​ad.php?t=991302

In practical terms, the full frame camera is capable of a shallower depth of field than the crop camera.

The reason is that as a photographer, you're generally interested in 4 things: distance to the subject, angle of view, amount of time captured, and depth of field.* Of those, the last is what we're considering in relationship to the others when comparing two cameras: a full frame camera and a 1.6x crop camera.

That means that you're going to try to set all the other things the same regardless of what camera you're using, and because of that the actual settings you use will differ between the two cameras. For instance, you control the angle of view by changing the focal length of the lens, but each camera will require a different setting from the other in order to achieve the same angle of view. In particular, the focal length on the full frame camera will have to be 1.6x that used on the crop camera. For instance, if the angle of view you're trying to achieve is such that you'd need to use 50mm on the crop camera, then you'd need to use 80mm on the full frame camera to achieve exactly the same angle of view.

Distance to the subject is, of course, camera-independent. So is shutter speed.

So now we're left with depth of field. Well, because the angle of view is fixed and forces you to use different focal length settings on the two cameras, the aperture you select will also be different between the two. In particular, the aperture you use on the crop camera will have to be wider than the aperture used on the full frame camera, by about 1 1/3 stops.


So what does that have to do with the depth of field potential of the cameras? Plenty. Because the lens selection for the full frame camera and crop camera are essentially the same, it means the widest aperture available is also roughly the same. This is primarily because both cameras use the same lens mount. In any case, because at a given angle of view the aperture on the crop camera has to be wider than on the full frame camera in order to achieve the same depth of field, it means that the crop camera will run out of aperture before the full frame camera will, and as a result it's possible to make the depth of field shallower on the full frame camera than on the crop camera for any given angle of view, assuming (of course) that the widest aperture available is roughly the same for both.


If that doesn't make any sense, just take away the answer, which is: for a given photographic scenario, the full frame camera will almost always be able to produce a shallower depth of field than the crop camera.

* Of course, as photographers we're also interested in achieving a certain tonal capture, which is governed by the aperture, shutter speed, and ISO settings chosen. Often, the exposure forces us to vary the aperture or shutter speed to some degree in order to achieve the tonal capture we're after.

There are three very different scenarios for comparing depth of field with two different format cameras.

NOTE: ALL of these scenarios assume that the same f-stop is used for all images.

The first scenario assumes that the two cameras are set up to make identically framed images with the cameras and subjects in the same positions. This means that one would use different focal lengths on the different format cameras that would frame the scene the same way. In this scenario, the larger format camera will have a noticeably shallower depth of field if the same f-stop were used for both cameras.

The second scenario also assumes that the two cameras are used from the same position but that the same focal length lenses are used on the two cameras. There is still a variable involved here that makes the depth of field different for the two format cameras, and this is the "circle of confusion". In this scenario, the image from the larger format camera would have a significantly deeper depth of field. Note that the two images would look very different because of the greater field (angle) of view with the larger format camera.

The third scenario also assumes that the two cameras are used with the same focal length lenses but the camera positions are different in order to frame the primary subject the same. In this scenario, the image from the larger format camera would have a shallower depth of field. Note that the two images would look different because of the different perspective (size relationship of objects at different distances from the camera) caused by the different distances between the camera and the primary subject.

To compare each scenario, you can use this on-line depth of field calculator to provide the detailed results for each setup.

In my opinion, the first scenario is the only practical way to compare two different format cameras for depth of field, as the resulting images will have the same perspective and framing.


Though I posted this in another thread that got totally out of hand, I hope it can just be used for what it is here.

Pasukun wrote in post #11727368
I just learned something new today, and I would like to verify this information with someone who has a good understanding of DOF.

I always thought FF had a shallower DOF than 1.6 crop, and it certainly is shallower if you are trying to maintain a same subject size to a frame size ratio, but if you ignore that ratio and keep the distance to the subject the same, then the 1.6 crop is in fact has a shallower DOF than FF, due to the smaller circle of confusion value on 1.6 crop.
...
Any remark would be appreciated.
Specially on the CoC calcuation.
Thanks.

Only one remark,
I'm glad I'm not soo lonely here anymore.
I did mentioned the same here several times, but POTN people are always recommending me to walk back twenty feet in ten feet long room. LOL.

It was a very good read, I would like to thank all of you guys for that.

I am normally not that mathematical or calculating, but I knew that FF had a shallower DOF than 1.6 crop, since none of the FOV gets chopped off in FF, so you get to stay closer to your subject using the same focal length. If you want to maintain the same composition between FF and 1.6 crop, you would have to either increase or decrease the distance to your subject or change the focal length entirely to match the FOV, and I have never questioned that fact. In fact, I thought FF had around 1 & 1/3 stops of DOF advantage over the 1.6 crop, due to that 1.6x multiplication.

I assumed, if the distance to the subject does not change and the focal length and aperture of the lens does not change, then the DOF must be the same since nothing really changed other than the sensor size, which was nothing more than a cropping. So when I was trying to convince my friend (who is engineer) to upgrade his camera to FF, he wanted to know the difference in exact numbers, and to my surprise, they were not same. Farther calculation had me even more puzzled, even though it was not practical.

In above scenario, for whatever reason you decided not to move and totally disregard the composition, but wanted to achieve same DOF, you would have to tighten down the aperture on 1.6 crop to f/3.2 while FF has it on f/2, then you would have about same DOF, according to the DOF calculation.
And that was uncalled for.
I always thought the DOF was driven by distance to the subject, focal length and aperture. Never anticipated the sensor size.

It still does not mean much thought.
I know just how much DOF advantage I can gain from using FF. And it does not take much to see the difference.

Nice to you learned and understand it, it's not really that hard. Have fun

Pasukun wrote in post #11730974
It was a very good read, I would like to thank all of you guys for that.

I am normally not that mathematical or calculating, but I knew that FF had a shallower DOF than 1.6 crop, since none of the FOV gets chopped off in FF, so you get to stay closer to your subject using the same focal length. If you want to maintain the same composition between FF and 1.6 crop, you would have to either increase or decrease the distance to your subject or change the focal length entirely to match the FOV, and I have never questioned that fact. In fact, I thought FF had around 1 & 1/3 stops of DOF advantage over the 1.6 crop, due to that 1.6x multiplication.

I assumed, if the distance to the subject does not change and the focal length and aperture of the lens does not change, then the DOF must be the same since nothing really changed other than the sensor size, which was nothing more than a cropping. So when I was trying to convince my friend (who is engineer) to upgrade his camera to FF, he wanted to know the difference in exact numbers, and to my surprise, they were not same. Farther calculation had me even more puzzled, even though it was not practical.

In above scenario, for whatever reason you decided not to move and totally disregard the composition, but wanted to achieve same DOF, you would have to tighten down the aperture on 1.6 crop to f/3.2 while FF has it on f/2, then you would have about same DOF, according to the DOF calculation.
And that was uncalled for.
I always thought the DOF was driven by distance to the subject, focal length and aperture. Never anticipated the sensor size.

It still does not mean much thought.
I know just how much DOF advantage I can gain from using FF. And it does not take much to see the difference.

That's because you left out the 4th variable: CoC.

CoC is your sensor size.

Willie wrote in post #11731036
CoC is your sensor size.

That's not quite correct. The CoC (Circle of Confusion) value is related to the film/sensor size, but is not the size of the film frame or sensor in itself.

Willie wrote in post #11731036
That's because you left out the 4th variable: CoC.

CoC is your sensor size.

Right. That was the whole purpose of that statement. Thx.

SkipD wrote in post #11731062
That's not quite correct. The CoC (Circle of Confusion) value is related to the film/sensor size, but is not the size of the film frame or sensor in itself.

I'm sure you knew what I meant. I was in a hurry.

EDIT: I guess I should have been more accurate, given the seemingly confusing nature of this topic.

Like Dr. Fraser Crane (of Cheers) said, "Inaccuracies are the sign of sloppy thinking".

Willie wrote in post #11731077
I'm sure you knew what I meant.

Yep. I sure did. However, the newbies reading these threads would not have.

SkipD wrote in post #11731142
Yep. I sure did. However, the newbies reading these threads would not have.

Hence my edit. I realized that after I typed my response.